The present invention relates generally to oscillators and more particularly to temperature compensated crystal oscillator arrangements.
Crystal oscillators are widely employed in electronic equipment, usually in situations where a precise frequency of oscillation is required. However, the natural resonant frequency of a crystal is a function of temperature, and for some applications the operating temperature range is so great having regard to the frequency stability requirements that special measures have to be taken to reduce the effects of temperature variations upon the operating frequency of the oscillator. This reduction can be achieved by maintaining the crystal at a constant temperature in a temperature controlled oven, but this requires a relatively large amount of space and involves the consumption of a relatively large amount of power. This is the approach used in the oven controlled crystal oscillator (OCXO). In a temperature compensated crystal oscillator (TCXO) an alternative approach to obtaining the required reduction in temperature sensitivity is obtained by the use of a variable reactance element in the feedback path of the oscillator to `pull` the forced resonant frequency of the oscillator in such a way as to compensate for the changes that would otherwise result from the changes in the natural resonant frequency of the crystal. Typically the variable reactance element of a TCXO is a varactor diode whose reactance is controlled by altering the d.c. voltage applied across its terminals. Various methods have been proposed for generating the requisite compensation voltage for the varactor diode, but to provide a reasonable stability over the temperature range from -40.degree. C. to +85.degree. C. the approach most commonly used to date is to provide a ladder network of three thermistors and five resistors. The values of the components in this latter are chosen having regard to the particular temperature characteristic exhibited by the uncompensated crystal, and this varies from oscillator to oscillator because the variations between nominally identical crystals prove to be quite significant. As a result, compensation with this form of ladder network typically requires the selection of appropriate values of resistors from three decades of 1% accuracy E 96 series resistors (resistors having 96 perferred values per decade). Since such highly precise resistors are often expensive, the cost of such known compensation network is considerable.